1. Field of the Invention
The present invention relates to an object detection apparatus, and more particularly to a technique for detecting an object to be detected from a frame image.
2. Description of the Background Art
There are object detection apparatuses each of which determines whether or not there is an object to be detected in an image captured by a camera, or the like. The object detection apparatus is mounted, for example, on a vehicle, together with a camera. The object detection apparatus can notify a driver that there is a pedestrian by determining whether or not there is a human figure in the image captured by the camera. By using the object detection apparatus, the driver can easily grasp the circumstances outside the vehicle.
The object detection apparatus uses pattern matching in order to determine whether or not there is an object to be detected in an image. As an algorithm of the pattern matching, a neural network, a support vector machine, and the like can be used. The object detection apparatus sets a region (window) in which an object is to be detected, in an inputted image. The object detection apparatus determines whether or not the window includes the object to be detected by using a program in which the above algorithm is implemented.
Japanese Patent Application Laid Open Gazettes Nos. 2008-21034 and 2009-70344 each disclose an object detection apparatus using pattern matching.
An apparatus disclosed in Japanese Patent Application Laid Open Gazette No. 2008-21034 detects a pedestrian from an image captured by a vehicle-mounted camera. Specifically, when the apparatus detects a candidate object which has a possibility of being a pedestrian from the image by using a neural network, the apparatus compares the candidate object with a head, limbs, and the like of a pedestrian, to thereby determine whether the candidate object is a pedestrian or not.
An apparatus disclosed in Japanese Patent Application Laid Open Gazette No. 2009-70344 sets a plurality of detection windows having respective regions for detection which partially overlap one another, in an image captured by a camera. The apparatus performs pattern matching on each of the detection windows by using a reference pattern of an object to be recognized (pedestrian or the like). For a region on which the pattern matching is performed a plurality of times, the apparatus integrates the results of a plurality of pattern matching operations. On the basis of the integral result, a position of the pedestrian is specified.
In order to detect an object to be detected from an image, the object detection apparatus using pattern matching learns a pattern of the object to be detected in advance by using an image (sample image data) including the object to be detected. If the object to be detected is a pedestrian, for example, the sample image data is generated by extracting a region including the pedestrian from a learning image in which the pedestrian is imaged. The learning image is captured under a predetermined exposure condition.
The object detection apparatus detects the pedestrian from a captured image inputted from the camera after learning the pattern of the pedestrian. There are some cases, however, where a detection accuracy of the pedestrian becomes lower because of disagreement between an exposure condition under which the captured image is generated and that under which the learning image is generated.
In a case, for example, where the object detection apparatus detects a pedestrian from a captured image taken in the shade while the learning image is generated by capturing an image in a sunny place, since the exposure condition for the captured image is different from that for the learning image, there is a possibility that the object detection apparatus cannot detect any pedestrian though the captured image taken in the shade includes a pedestrian.
Therefore, it is desirable that the captured image should be generated under the same exposure condition as that under which the learning image is generated, in order to improve the accuracy for detecting an object to be detected from a captured image. In such a case, there is a possible method in which the object detection apparatus controls the exposure of the camera in conformity with the exposure condition for the learning image. In this method, the object detection apparatus analyzes the captured image inputted from the camera and feeds the exposure condition back to the camera. If the camera does not accept the exposure condition from the outside, however, this method cannot be used.